The present invention relates to a method and a device for selective catalytic reduction of NOx levels in oxygen-containing exhaust gases using ammonia and a reduction catalyst.
Ammonia is known as a selective and effective reduction agent for the catalytic reduction (SCR) of nitrogen oxides in oxygen-containing exhaust gases of internal combustion engines, for example.
For reasons of toxicity and safety problems arising from storing gaseous ammonia, in particular, procedures whereby ammonia is generated at the site of its use as a reduction agent by hydrolysis of urea have been developed.
European Patent No. 0 487 886 describes a method of quantitative hydrolysis of urea wherein an aqueous urea solution is sprayed onto an evaporator and forwarded via a hydrolysis catalyst with the surfaces of the evaporator and hydrolysis catalyst being coated with active components that catalyze the quantitative hydrolysis of urea into ammonia and CO2 and inhibit the formation of solid urea reaction products.
The above-mentioned method has the disadvantage that a liquid must be added for preparing the reaction medium, and this liquid cannot be metered as accurately as a gaseous medium. Therefore, the gas stream that leaves the reduction catalyst usually still contains unreacted ammonia, which cannot be simply released into the atmosphere, but must be decomposed into non-toxic components using an oxidation catalyst.
The procedure is associated with high equipment costs due to the hydrolysis catalyst, as well as the transport of urea and its introduction, which must be uniform over the cross section of the flow.
The hydrolysis catalyst has the additional disadvantage that it absorbs ammonia at low temperatures. If the temperature rises quickly, most of the absorbed ammonia is released and can no longer be used as a reduction agent for selective catalytic reduction.
The use of an aqueous urea solution causes other problemsxe2x80x94in winter operation of motor vehicles and due to the drop in exhaust gas temperature because of water evaporating in the exhaust gas: the 30%-35% aqueous urea solution used in the related art has a freezing point of about xe2x88x9211xc2x0 C. At lower temperatures, particularly at the freezing point of diesel fuel, the operation of the motor vehicle is no longer ensured. While the freezing point can be lowered using additives, additives such as ammonium formiate, for example, are usually particularly corrosive, so that their use poses new problems.
The exhaust gas is cooled by about 20 K due to the need of evaporating the water it contains. The unsatisfactory low-temperature performance of SCR catalysts is particularly strongly manifested in this case.
The method according to the present invention has the advantage over the related art that a gaseous reduction agent is provided, which can be accurately metered, causes no problems in winter operation and thus requires no additional antifreeze measures; the problem of poor low-temperature performance of SCR catalysts is thus somewhat alleviated with its use.
In addition, the device for carrying out the method according to the present invention is relatively simple, since no urea hydrolysis catalyst is used, in addition to being compact due to the fact that ammonia in the solid storage medium according to the present invention takes up 3 times less space than the aqueous urea solution. The space thus freed up can possibly be used for an additional SCR unit.
The requirements for the metering system are less stringent, since, in contrast with a urea-water solution, clogging problems in the metering components cannot occur.
There is no more need for compressed air support for introducing the reduction agent, since a sufficient pressure gradient with respect to the exhaust system is always created when ammonia is released. The system is therefore equally well suited for use in passenger cars and utility vehicles.
In contrast with methods using gaseous ammonia, safety problems are alleviated, since the ammonia carried in the vehicle is mostly bound to the storage substance, i.e., it is not freely available.
Furthermore, it is particularly advantageous that the solid storage media used according to the present invention are regeneratable, i.e., the emptied container can be refilled with ammonia. The number of regeneration cycles may be as high as 1000.
Strontium chloride SrCl2, which has a high storage density for ammonia, comparable to the storage density of solid urea, can be advantageously used as the solid storage medium. Strontium chloride can be replaced totally or in part by calcium chloride, which offers comparable advantages.
It is particularly advantageous to use the waste heat of the engine coolant and/or the exhaust gas to heat the solid storage medium.
The device according to the present invention can be advantageously equipped with a buffer container for gaseous ammonia in order to have gaseous ammonia available even in intermittent engine operation and/or in start phases, regardless of the heating phase of container 1, when there is insufficient energy for heating the ammonia storage medium.